(1) Field of the Invention
The present invention relates to a magnetic recording medium, and more particularly to a magnetic recording medium wherein a radiation-curable compound is used as a binder and which improves durability.
(2) Description of the Prior Art
As a method of producing a magnetic recording media generally used widely, there has been a method in which at least one thermoplastic resin such as vinyl chloride-vinyl acetate resin, vinyl chloride-vinylidene chloride resin, cellulose resin, acetal resin, urethane resin, acrylonitrile-butadiene resin, etc. is used, either singly or in combination, as a binder. This method, however, has the drawback that a magnetic layer obtained has poor abrasion resistance, resulting in contamination of a feed passage of a magnetic tape.
There has also been known a method wherein a thermoplastic resin, such as melamine resin, and a urea resin is used; or a method in which a binder capable of being crosslinked by a chemical reaction, for instance, an isocyanate compound or an epoxy compound is added to the thermoplastic resin. The use of the crosslinkable binder, however, has the following drawbacks (1) and (2).
(1) The storage stability of a resin solution having a magnetic powder dispersed therein is poor, namely, the pot life of the solution is short, and it is therefore impossible to maintain uniform physical properties of a magnetic coating liquid and hence the uniform quality of magnetic tapes. PA1 (2) A heat treatment step is required for curing a coating after the coating and drying steps, and it takes a long time to obtain a finished product. PA1 (a) First, a di-isocyanate compound and a hydroxyalkyl (meth)acrylate compound are mixed with each other and stirred. In this case, it is preferable to add a polymerization inhibitor such as hydroquinone and to set the temperature to 70.degree. C. or lower. PA1 (b) Then, an inert organic solvent such as methyl ethyl ketone is added to the reaction system, further a dialkanolamine is added to the system, and the resultant mixture is stirred. In this case it is preferable to add the dialkanolamine dropwise to the system and to set the temperature of the system to 10.degree. C. or lower.
As a means for overcoming these drawbacks, a method of producing magnetic media in which acrylic acid ester oligomer and monomer are used as a binder and curing after drying is effected by electron beams is disclosed in Japanese patent publication No. 47-12423 (1972), Japanese Patent Application Laid-Open (KOKAI) Nos. 47-13639 (1972), 47-13104 (1972), 50-77433 (1975) and 57-130229 (1982), etc. According to the disclosed method, however, it has been impossible to obtain a magnetic recording medium having high electrical characteristics, mechanical properties or durability.
Recently, there has been a particular demand for a high degree of electrical characteristics, and there has therefore been an increasing demand for a magnetic coating liquid which provides good dispersion properties of the ferromagnetic powder. If the dispersion properties of the ferromagnetic powder are poor, a lowering of output or generation of noise would result.
Besides, the conventional method in which electron beam curing is used has the drawback that when the content of a magnetic substance is increased for high-density recording, the storage stability of the magnetic coating liquid becomes poor and the electrical characteristics of the magnetic tape obtained are unsatisfactory for practical use. In addition, for prolonging the recording time of the magnetic tape, it is necessary that the support be thinner and the mechanical properties of the magnetic layer be improved.
Thus, with the recent wide spread of magnetic recording media such as video tapes, floppy disks, etc., severe use conditions have come to be imposed on the magnetic recording media, but a magnetic layer provided with mechanical properties sufficient for the severe conditions has not been attained.